The present invention relates to a steel sheet suitably used for reducing the weight of an automobile and enhancing the safety of passengers in the case of collision.
In order to suppress a discharge of carbonic acid gas from an automobile engine, the weight of an automobile body has been reduced by using high strength steel sheets when the automobile body is manufactured. Further, in order to ensure the safety of passengers, investigations have been made so that not only mild steel sheets but also high strength steel sheets can be used for automobile bodies. However, the high strength steel sheet is inferior to the mild steel sheet from the viewpoint of strain rate dependency. The problem with the high strength steel sheet is described below. In the case where the high strength steel sheet is deformed at high rate of deformation corresponding to the collision of an automobile, a difference between the flow stress of the high strength steel sheet and the flow stress of the mild steel sheet becomes smaller than that in the case of static deformation conducted in a conventional tensile strength test. Therefore, the advantage of using the high strength steel sheet, the static strength of which is higher than that of the mild steel sheet, is decreased. Of course, it is very important to develop steel sheets, which has an excellent strain rate sensitivity of the flow stress, so that the safety of passengers in the case of collision and the reduction of the weight can be made compatible with each other.
In order to make up for the deterioration of the strain rate sensitivity, the present inventors took into account of the manufacturing process of an actual impact absorbing member including a press forming process, coating process and baking finishing process, and developed steel sheets, the dynamic strength of which was high after the actual impact absorbing member had gone through these processes. This technique is disclosed in Japanese Unexamined Patent Publication Nos. 9-287050 and 9-296247.
When a strain rate sensitivity of a steel sheet is not lowered in spite of an increase of a strength of the steel sheet is increased, the impact absorbing characteristic can be enhanced. However, this problem has seldom been tackled until now.
For example, Japanese Unexamined Patent Publication No. 6-322476 discloses a steel sheet for automobile use, the impact resistance of which is high, which is produced by reducing the quantities of C and N existing a state of a solid solution. According to this patent publication, only the following is disclosed. A ratio of the yield strength in the case of quasi-static deformation to the yield strength in the case of dynamic deformation is enhanced, which is shown on line 31 to 38 in the third column of page 3 in the patent publication. However, there is no description about enhancing the tensile strength in the case of deformation conducted at high strain rate.
In this connection, quasi-static deformation (static strength) is defined as deformation (strength) at the strain rate of 10xe2x88x923/sec in the case of a conventional tensile test, and high strain rate deformation or dynamic deformation (dynamic strength) is defined as deformation (strength) at the strain rate of 103/sec.
Strain rate sensitivity is defined as a difference ("sgr"dxe2x88x92"sgr"s) between the average stress "sgr"d from the nominal strain 5% to 10% in the case of deformation conducted at the strain rate of 103/sec and the average stress "sgr"s in the case of deformation conducted at the strain rate of 103/sec.
For example, as described in E. Nakanishi et. al., Structural Failure, Product Liability and Technical Insurance, IV (1992), 423, Elsevier, the strain rate sensitivity of then conventional steel sheet for automobile use is deteriorated regardless an increase in the strength, and the enhancement of the impact absorbing capacity is limited.
It is an object of the present invention to provide a ferritic steel sheet containing Co and Cr in the state of solid solution so that the static strength of a conventional steel sheet can be increased and further the enhancement of the dynamic strength cannot be decreased.
According to conventional knowledge, it is inevitable that when the strength of a steel sheet is increased, the strain rate sensitivity is deteriorated as compared with a mild steel sheet. Therefore, the effect provided by using a high strength steel sheet is reduced. However, in order to cope with the strict requirements against collision damage and in order to improve the fuel consumption, it is necessary to provide a fundamental solution.
To solve the above problems, the present inventors made investigation into the fundamental theory of deformation of steel so that the increase of strength can be compatible with the strain rate sensitivity. They researched the action and effect of elements contained in the material in the state of solid solution. As a result of the research, the present inventors found that Co and/or Cr, which exist in the state of solid solution in the ferrite phase, which are conventionally considered to have little effect on the static strength, that is, which are conventionally considered to have little capacity of enhancing the strength by forming solid solution, exert an important action on the strain rate sensitivity in the process of deformation conducted at a high strain rate.
The present invention has been accomplished the invention according to the above knowledge. The summary of the present invention is described as follows.
(1) A ferritic steel sheet excellent at strain rate sensitivity characterized in that: Co and/or Cr are contained by not less than 0.01 mass % and not more than 4.0 mass % in total in the state of solid solution in the ferrite phase of steel.
(2) A ferritic steel sheet excellent at strain rate sensitivity comprising; in terms of mass %, C: not less than 0.0001% and not more than 0.05%, Si: not less than 0.01% and not more than 1.0%, Mn: not less than 0.01% and not more than 2.0%, P: not more than 0.15%, S: not more than 0.03%, Al: not less than 0.01% and not more than 0.1%, N: not more than 0.01%, and O: not more than 0.007%, wherein Co and/or Cr are contained by not less than 0.01% and not more than 4.0% in total in the solid solution in the ferrite phase, and the remainder Fe is and unavoidable impurities.
(3) A ferritic steel sheet excellent at strain rate sensitivity according to item (2), further comprising; in terms of mass %, at least one or two of the elements of Ti: not more than 0.20%, Nb: not more than 0.20% and B: not more than 0.005%.
(4) A ferritic steel sheet excellent at strain rate sensitivity according to item (3), further comprising; in terms of mass %, at least one or two of the elements of Mo: not more than 1.0%, Cu: not more than 2.0% and Ni: not more than 1.0%.
(5) A ferritic steel sheet excellent at strain rate sensitivity comprising; in terms of mass %, C: not less than 0.05% and not more than 0.25%, Si: not less than 0.01% and not more than 2.5%, Mn: not less than 0.01% and not more than 2.5%, P: not more than 0.15%, S: not more than 0.03%, Al: not less than 0.01% and not more than 1.0%, N: not more than 0.01%, and O: not more than 0.007%, wherein Co and/or Cr are contained by not less than 0.01% and not more than 4.0% in total in the solid solution in the ferrite phase, and the remainder Fe and unavoidable impurities.
(6) A ferritic steel sheet excellent at strain rate sensitivity according to item (5), further comprising; in terms of mass %, at least one or two of the elements of Ti: not more than 0.20%, Nb: not more than 0.20%, V; not more than 0.20% and B: not more than 0.005%.
(7) A ferritic steel sheet excellent at strain rate sensitivity according to item (6), further comprising; in terms of mass %, at least one or two of the elements of Mo: not more than 1%, Cu: not more than 2% and Ni: not more than 1%.
(8) A ferritic steel sheet excellent at strain rate sensitivity according to one of items (1) to (7), wherein plating is conducted on the steel sheet.
(9) An automobile characterized in that: at least one or two of the cross member, front side member, center pillar, rocker, side roof rail and rear side member are made of a ferritic steel sheet described in one of items (1) to (8).
In the present invention, the ferritic steel sheet is defined as a hot-rolled steel sheet and a cold-rolled steel sheet except for a stainless steel sheet of austenitic and a ferritic stainless steel sheet.
In the present invention, Co contained in steel in the state of solid solution is defined as a quantity of Co obtained when the quantity of Co contained in steel in the form of carbide and nitride is subtracted from the total quantity of Co contained in steel. Also, Cr contained in steel in the state of solid solution is defined as a quantity of Cr obtained when Cr contained in steel in the form of carbide and nitride is subtracted from the total Cr content contained in steel. The total amount of each element contained in steel is found by the real time quantive analysis method, and the quantity existing in the form of carbide or nitride is found in such a manner that carbide, nitride or impurities is extracted by the electrolytic extraction method, and the thus extracted one is fused by acid or alkali and then subjected to ICP (high frequency induction plasma conbination analyzer).